1. Field of the Invention
This invention relates to a process for reducing emissions of carbon monoxide and sulfur oxides from the regenerator of a catalytic cracking unit. More particularly, the invention relates to the regeneration of deactivated cracking catalyst in the presence of a minor amount of palladium and ruthenium.
2. Description of the Prior Art
A major industrial problem involves the development of efficient methods for reducing the concentration of air pollutants, such as carbon monoxide and sulfur oxides, in waste gas streams which result from the processing and combustion of sulfur and carbon containing fuels. The discharge of these waste gas streams into the atmosphere is environmentally undesirable at the sulfur oxide and carbon monoxide concentrations which are frequently encountered in conventional operations. The regeneration of cracking catalyst, which has been deactivated by coke deposits in the catalytic cracking of sulfur-containing hydrocarbon feedstocks, is a typical example of a process which can result in a waste gas stream containing relatively high levels of both carbon monoxide and sulfur oxides.
Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels utilized by internal combustion engines. In fluidized catalytic cracking processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated transfer line reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
In the catalytic cracking of hydrocarbons, some nonvolatile carbonaceous material or coke is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons and generally contains from about 4 to about 10 weight percent hydrogen. When the hydrocarbon feedstock contains organic sulfur compounds, the coke also contains sulfur. As coke accumulates on the cracking catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stocks diminishes.
Catalyst which has become substantially deactivated through the deposit of coke is continuously withdrawn from the reaction zone. This deactivated catalyst is conveyed to a stripping zone where volatile deposits are removed with an inert gas at elevated temperatures. The catalyst particles are then reactivated to essentially their original capabilities by substantial removal of the coke deposits in a suitable regeneration process. Regenerated catalyst is then continuously returned to the reaction zone to repeat the cycle.
Catalyst regeneration is accomplished by burning the coke deposits from the catalyst surfaces with an oxygen containing gas such as air. The combustion of these coke deposits can be regarded, in a simplified manner, as the oxidation of carbon according to the following equations: EQU C+O.sub.2 .fwdarw.CO.sub.2 ( 1) EQU 2C+O.sub.2 .fwdarw.2CO (2) EQU 2CO+O.sub.2 .fwdarw.2CO.sub.2 ( 3)
Reactions (1) and (2) both occur under typical catalyst regeneration conditions wherein the catalyst temperature usually ranges from about 565.degree. to about 815.degree. C. The combustion of carbon monoxide to carbon dioxide according to reaction (3) proceeds only at temperatures above about 595.degree. C. Consequently, the incomplete combustion of carbon monoxide during catalyst regeneration can result in significant concentrations of carbon monoxide in the regeneration zone effluent gas. The discharge of this carbon monoxide into the atmosphere is undesirable, not only from an environmental point of view, but also because it represents a wasted source of heat energy. The combustion of carbon monoxide yields approximately 4,350 B.T.U. per pound.
When sulfur-containing feedstocks, such as petroleum hydrocarbons containing sulfur compounds, are utilized in a catalytic cracking process, the coke deposited on the catalyst contains sulfur. During regeneration of the coked deactivated catalyst, the coke is burned from the catalyst surfaces which results in the conversion of the sulfur to sulfur dioxide together with small amounts of sulfur trioxide. This burning can be represented, in a simplified manner, as the oxidation of sulfur according to the following equations: EQU S(in coke)+O.sub.2 .fwdarw.SO.sub.2 ( 4) EQU 2SO.sub.2 +O.sub.2 .fwdarw.2SO.sub.3 ( 5)
The removal of carbon monoxide from the waste gas produced during the regeneration of deactivated cracking catalyst can be accomplished by conversion of the carbon monoxide to carbon dioxide in a separate zone or carbon monoxide boiler after separation of the regeneration zone effluent gas from the catalyst. This approach is described, for example, in U.S. Pat. No. 2,753,925 to Campbell et al. Such methods, however, require complex auxiliary equipment which serves to increase operating and capital costs.
An alternative approach to the control of carbon monoxide emissions from the regeneration of cracking catalyst is set forth in U.S. Pat. No. 3,909,392 to Horecky et al. This patent discloses a process wherein the essentially complete combustion of carbon monoxide to carbon dioxide is carried out within the regeneration zone with recovery of the resulting heat by direct transfer to the catalyst particles. In addition, the patent also teaches the use of combustion catalysts within the regeneration zone to promote the combustion of carbon monoxide. These combustion catalysts include a metallic bar, mesh network, or screen in the regeneration zone; and fluidizable metal compounds, particularly powdered oxides of transition group metals such as ferric oxide, manganese dioxide and rare earth oxides. This approach not only substantially eliminates carbon monoxide emissions, but also permits the preparation of regenerated catalyst having an extremely low content of residual coke, reduces or eliminates the need to preheat the hydrocarbon feedstock, and results in improved yields of more valuable products.
British Pat. No. 1,499,682 is directed to a catalytic cracking process involving the use of a cracking catalyst in association with a carbon monoxide oxidation promoter which is a metal having an atomic number of at least 20. Among others, it is disclosed that platinum, palladium and rhodium are active oxidation promoters.
German Offenlegungsschrift No. 2,444,911 and its counterparts U.S. Pat. Nos. 4,072,600; 4,088,568 and 4,093,535 to Schwartz, disclose that a cracking catalyst containing less than 100 parts per million, calculated as metal and based on total catalyst, of at least one metal component selected from the group consisting of the metals of Periods 5 and 6 of Group VIII of the Periodic Table and rhenium or their compounds is effective in reducing the carbon monoxide content of effluent gas derived from the regeneration of cracking catalyst. Similarly, U.S. Pat. No. 4,064,037 to Graven et al., U.S. Pat. No. 4,064,039 to Penick and U.S. Pat. No. 4,107,032 to Chester also disclose the use of platinum group metals and rhenium to promote the combustion of carbon monoxide within the regeneration zone of a fluidized catalytic cracking unit.
One approach to the removal of sulfur oxides from a waste gas stream involves scrubbing the gas with an inexpensive alkaline material, such as lime or limestone, which reacts chemically with the sulfur oxides to give a nonvolatile product which is discarded. Unfortunately, this approach requires a large and continual supply of alkaline scrubbing material, and the resulting reaction products can create a solid waste disposal problem of substantial magnitude. In addition, this approach requires complex and expensive auxiliary equipment.
A second approach to the control of sulfur oxide emissions involves the use of sulfur oxide absorbents which can be regenerated either thermally or chemically. An example of this approach to the removal of sulfur oxides from the regeneration zone effluent gas stream in a cyclic, fluidized, catalytic cracking process is set forth in U.S. Pat. No. 3,835,031 to Bertolacini et al. This patent discloses the use of a zeolite-type cracking catalyst which is modified by impregnation with one or more metal compounds of Group IIA of the Periodic Table, followed by calcination, to provide from about 0.25 to about 5.0 weight percent of Group IIA metal or metals as an oxide or oxides. The metal oxide or oxides react with sulfur oxides in the regeneration zone to form nonvolatile inorganic sulfur compounds. These nonvolatile inorganic sulfur compounds are then converted to the metal oxide or oxides and hydrogen sulfide upon exposure to hydrocarbons and steam in the reaction and stripping zones of the process unit. The resulting hydrogen sulfide is disposed of in equipment which is conventionally associated with a fluidized catalytic cracking process unit. Belgian Pat. No. 849,637 is also directed to a process wherein a Group IIA metal or metals are circulated through a cyclic fluidized catalytic cracking process with the cracking catalyst in order to reduce the sulfur oxide emissions resulting from regeneration of deactivated catalyst.
U.S. Pat. No. 4,153,534 to Vasalos discloses a process similar to that set forth in U.S. Pat. No. 3,835,031, which involves the removal of sulfur oxides from the regeneration zone flue gas of a cyclic, fluidized, catalytic cracking unit through the use of a zeolite-type cracking catalyst in combination with a regenerable sulfur oxide absorbent which absorbs sulfur oxides in the regeneration zone and releases the absorbed sulfur oxides as a sulfur-containing gas in the reaction and stripping zones of the process unit. The sulfur oxide absorbent comprises at least one free or combined element selected from the group consisting of sodium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, the rate earth metals and lead.
U.S. Pat. No. 4,071,436 to Blanton et al. teaches that alumina and/or magnesia can be used to absorb sulfur oxides from a gas at a temperature in the range from 1000.degree. to 1500.degree. F. and the absorbed sulfur oxides can be removed by treatment with a hydrocarbon at a temperature in the range from 800.degree. to 1300.degree. F. It is further disclosed that sulfur oxide emissions from the regeneration zone of a cyclic, fluidized, catalytic cracking unit can be reduced by combining alumina and/or magnesia with the hydrocarbon cracking catalyst. Similarly, U.S. Pat. No. 4,115,249 to Blanton et al. teaches that a cracking catalyst can be impregnated with an aluminum compound and utilized in a cyclic, fluidized, catalytic cracking process for the purpose of reducing regenerator sulfur oxide emissions.
U.S. Patent Application Ser. No. 29,264 by Bertolacini et al. discloses a process for the removal of sulfur oxides from a gas by an absorbent comprising at least one inorganic oxide selected from the group consisting of the oxides of aluminum, magnesium, zinc, titanium and calcium in association with at least one free or combined rare earth metal selected from the group consisting of lanthanum, cerium, praseodymium, samarium and dysprosium, wherein the ratio by weight of inorganic oxide or oxides to rare earth metal or metals is from about 0.1 to about 30,000. Absorbed sulfur oxides are recovered as a sulfur-containing gas comprising hydrogen sulfide by contacting the spent absorbent with a hydrocarbon in the presence of a hydrocarbon cracking catalyst at a temperature from about 375.degree. to about 900.degree. C. It is further disclosed that the absorbent can be circulated through a fluidized catalyst cracking process together with the hydrocarbon cracking catalyst to reduce sulfur oxide emissions from the catalyst regeneration zone.
An approach to the simultaneous control of carbon monoxide and sulfur oxide emissions from the regeneration of cracking catalyst is set forth in U.S. Pat. No. 4,153,535 to Vasalos et al. This patent discloses the circulation of a sulfur oxide absorbent through the catalytic cracking process with cracking catalyst and regeneration of the cracking catalyst in the presence of a metallic promoter. The promoter comprises at least one free or combined metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, platinum, vanadium, tungsten, uranium, zirconium, rhenium and silver. The sulfur oxide absorbent comprises at least one free or combined element which is selected from the group consisting of sodium, magnesium, calcium, strontium, barium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, lead and the rare earth metals. The metallic promoter serves to enhance the ability of the absorbent to absorb sulfur oxides in the regeneration zone of a cyclic, fluidized, catalytic cracking unit and also serves to enhance the combustion of carbon monoxide within the catalyst regeneration zone.
Similarly, U.S. Pat. No. 4,146,463 to Radford et al. discloses a process wherein a separately generated waste gas containing sulfur oxides and/or carbon monoxide is conveyed to the regeneration zone of a cyclic, fluidized, catalytic cracking unit where these pollutants are removed by contact with a sulfur oxide absorbent and an oxidation promoter, wherein the absorbent is a metal oxide which reacts with the sulfur oxides to form nonvolatile inorganic sulfur compounds and the promoter consists of at least one free or combined metallic element selected from Groups IB, IIB and III-VIII of the Periodic Table.
U.S. Pat. No. 4,115,250 to Flanders et al. describes the use of alumina to absorb sulfur oxides in the regeneration zone of a fluidized catalytic cracking unit while the combustion of carbon monoxide is simultaneously catalyzed by an oxidation promoter comprising a metal or compound of a metal selected from platinum, palladium, iridium, rhodium, osmium, ruthenium, copper and chromium.
The use of combustion promoters, particularly trace amounts of platinum, has recently found wide acceptance in the refining industry for the purpose of enhancing the combustion of carbon monoxide within the catalyst regeneration zone of a fluidized catalytic cracking unit. Unfortunately, the more active combustion promoters such as platinum and palladium also serve to promote the formation of nitrogen oxides in the regeneration zone. Since the discharge of nitrogen oxides into the atmosphere is environmentally undesirable, the use of these promoters has the effect of substituting one undesirable emission for another. In a conventional process for the regeneration of cracking catalyst which does not involve any significant carbon monoxide combustion, the regeneration zone effluent gas will contain from about 6 to 12 volume percent of carbon monoxide and from less than about 10 parts per million by volume (ppmv) to about 40 ppmv of nitrogen oxides. For a hydrocarbon feedstock containing about 0.08 weight percent nitrogen, the nitrogen oxide content of the regeneration zone flue gas will ordinarily be less than 10 ppmv when conventional regeneration is employed. The incorporation of 0.1 parts per million by weight (ppm) of platinum into the cracking catalyst can increase the nitrogen oxide content of the regeneration zone flue gas for this feedstock to the 100-200 ppmv range, and 1.5 ppm of platinum can increase the nitrogen oxide levels as high as about 900 ppmv.
If a platinum or palladium combustion promoter is employed solely for the purpose of enhancing the combustion of carbon monoxide within the catalyst regeneration zone of a fluidized catalytic cracking unit, the resulting increase in formation of nitrogen oxides, although undesirable, may be tolerable because of the very small amount of promoter required. However, our tests show that if the promoter is additionally used to enhance the ability of a sulfur oxide absorbent to absorb sulfur oxides in the regeneration zone, larger amounts of the promoter are frequently required and can result in the formation of intolerable amounts of nitrogen oxides.
Two nitrogen oxides, nitric oxide (NO) and nitrogen dioxide (NO.sub.2) are formed during the regeneration of cracking catalyst, and both are significant air pollutants. The U.S. Occupational Safety and Health Administration considers these oxides to be health hazards at 25 ppmv for NO and 5 ppmv for NO.sub.2. In addition, both oxides have been implicated in the formation of smog by photochemical reaction with hydrocarbons in the atmosphere. The term NO.sub.x, in common practice, refers to the sum of NO plus NO.sub.2.
When nitrogen containing feedstocks, such as petroleum hydrocarbons which contain organic nitrogen compounds, are used in a catalytic cracking process, the coke deposited on the catalyst contains nitrogen. This nitrogen in the coke represents one possible source of NO.sub.x in the regeneration zone effluent gas stream. In addition, molecular nitrogen from the combustion air represents a second possible source. On the basis of thermodynamic considerations, however, it is probable that organic nitrogen compounds in the coke will react with oxygen much more readily than molecular nitrogen to form NO.sub.x. At regeneration zone temperatures of about 705.degree. C., thermodynamic equilibrium calculations demonstrate that less than 100 ppmv of NO.sub.x can be attributed to the oxidation of molecular nitrogen. Since the use of a platinum combustion promoter can afford a regeneration zone effluent gas containing as much as 900 ppmv of NO.sub.x, it appears that a major portion of the NO.sub.x produced in the presence of platinum originates with the nitrogen in coke.
An article entitled "Reduction of Nitric Oxide by Monolithic-Supported Palladium-Nickel and Palladium-Ruthenium Alloys" by Bartholomew in Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 1, 1975, at pages 29-33, is addressed to an evaluation of the performance of monolithic-supported palladium-nickel and palladium-ruthenium alloys as catalysts for the removal of NO.sub.x from a reducing gas such as the exhaust gas from an internal combustion engine. This reference does not, however, suggest that a combination of palladium and ruthenium could be utilized to reduce or otherwise affect the NO.sub.x emissions produced in an oxidizing environment of the type found in the regeneration zone of a fluidized catalytic cracking unit. In addition, the reference is directed to the treatment of a preformed waste gas stream and does not suggest the desirability of incorporating a mixture of palladium and ruthenium into a combustion zone, such as the regenerator of a catalytic cracking unit, wherein a waste gas stream is generated in the presence of the palladium-ruthenium combination.